We have properties on objects that are supposed to be set only once, when first reading an object back from the database. Once in memory, they are not supposed to be set again, and attempting to do so is supposed to throw an exception. Unfortunately, hibernate's behavior appears to be less predictable than we had thought. It assigns properties more than once. For example, when merging a disconnected object, it loads one from the database (thus setting the properties), and then sets them again from the other object, thus violating the set-once rule. There are other cases similar to this. We modified our rule to allow setting to null (or some other value indicating not-yet-set) as many times as desired, then setting to some actual value, and allowing setting to that same value as many times as desired. Setting to a different value once set throws an exception. This seems to pacify hibernate, but we're not certain that it will work in all cases, and the behavior of hibernate as far as resetting properties is concerned is undefined. Our request is for hibernate to be enhanced to have a notion of set-once, which would be stated in the HBM file. This would make a property immutable, with the sole setting of that property to be done on reading in from the database. On save, a set-once property would be assumed to have already been set--hibernate would not set it itself when saving, only when reading.